I found these tiny anemones growing on my rock. At first I thought it was good, but within a day they had covered just about all the rocks. I realized it is Majano Anemone, and is considered a pest because it takes over the whole tank.

I had to take up each rock and take out each and every one. It was impossible to count them afterwords, but there sure must have been 150 of them, if not more.